1. Field of the Invention
The invention relates to a control device of a vehicle drive-train system including a lock-up clutch, and is particularly concerned with control associated with engagement of the lock-up clutch.
2. Description of the Related Art
A vehicle drive-train system is known which includes an engine, an electronic throttle valve operable to change the output rotational speed of the engine based on an electric command, without depending on an operation on an accelerator pedal, an automatic transmission having a manual shift mode in which shifting is effected based on a shift operation performed on a manually operated member, a torque converter provided between the automatic transmission and the engine, and a lock-up clutch operable to directly connect an input member and an output member of the torque converter with each other. Examples of the vehicle drive-train system of this type are described in Japanese Patent Application Publication No. 2006-153225 (JP-A-2006-153225), Japanese Patent Application Publication No. 2008-106841 (JP-A-2008-106841), and Japanese Patent Application Publication No. 2001-304003 (JP-A-2001-304003).
In the vehicle drive-train system as described above, during coasting, if lock-up control for engaging the lock-up clutch or lock-up slip control for partially engaging the lock-up clutch is finished because a tip-in operation of the accelerator pedal is performed or it is determined to prevent deterioration of a catalyst in an engine exhaust pipe, for example, the lock-up control or lock-up slip control will not be executed until subsequent depression of the accelerator pedal makes the input rotational speed of the torque converter close to the output rotational speed thereof. In the case where the input rotational speed of the torque converter (i.e., the engine speed) becomes lower than the output rotational speed thereof upon termination of the lock-up clutch (or lock-up slip control), if the lock-up clutch is executed under a condition where there is a large difference between the input rotational speed and the output rotational speed, it may be difficult to raise the input rotational speed due to an insufficient torque capacity of the lock-up clutch, or large shock may occur due to the rise or increase of the input rotational speed, or the amount of heat generated in the lock-up clutch may exceed an allowable value.
Accordingly, if the lock-up control (or lock-up slip control) is not executed irrespective of the driver's request for engine braking, as represented by the amount of operation of the accelerator pedal being equal to or close to zero, the engine is not brought into a driven state, and a sufficient engine brake, or a sufficient deceleration, cannot be obtained. Thus, even in the case where the driver performs a manual operation to effect a downshift in an attempt to provide a sufficient deceleration, the lock-up control is not executed since the situation where there is a large difference between the input rotational speed and output rotational speed of the torque converter does not change, as a result, a sufficient deceleration cannot be achieved. Also, if the lock-up control is not performed, it takes time to establish a condition in which the driving wheels are driven by the engine when the vehicle is accelerated again, namely, a condition in which the input rotational speed of the torque converter becomes higher than the output rotational speed thereof, resulting in a poor response.
According to a technology disclosed in JP-A-2008-106841, when a downshift is executed during coasting, the engine speed is temporarily raised or increased in a condition where the automatic transmission is placed in a neutral condition, and a clutch for shifting is engaged at the time when the output rotational speed becomes equal to a given synchronous rotational speed. Then, a given standby pressure is supplied to a hydraulic actuator of the lock-up clutch during the shift operation, and the lock-up clutch is engaged after completion of the shift operation. In this type of vehicle drive-train system, however, the engine speed that has been raised during the shift operation may be reduced at the time of completion of the shift operation; therefore, the difference between the input rotational speed and output rotational speed of the torque converter may be increased, and the lock-up control is not always performed with sufficient stability.
According to a technology disclosed in JP-A-2001-304003, when a difference between the input rotational speed and output rotational speed of the torque converter exceeds a specified range, the engine speed is increased so as to surely engage the lock-up clutch. In this type of vehicle drive-train system, however, the operation to increase the engine speed is performed after an operation to engage the lock-up clutch is started, though the operation to engage the lock-up clutch is not necessarily executed according to the difference between the input rotational speed and output rotational speed of the torque converter; therefore, the difference between the input rotational speed and output rotational speed of the torque converter may remain large due to time lag of the engine power output or the like when the lock-up clutch is engaged, and the lock-up control is not always performed with sufficient stability.